1. Field of the Invention
The present invention relates to an identification mark. In particular, the present invention relates to a method of forming and recognizing an identification mark for read-only memory (ROM), the application of which reduces the number of masks needed in the manufacturing process, thereby reducing manufacturing cost.
2. Description of the Related Art
Conventionally, in the manufacturing process of mask read-only memory (hereinafter referred to as a MASK ROM) cells, there is an identification mark formed on each chip having MASK ROM cells to easily distinguish each client""s product or products with different program codes. However, in the conventional method of manufacturing MASK ROM cells, it is necessary to prepare extra mark masks having different identification mark patterns for each chip in addition to code masks to code the MASK ROM cells by ion implantation. Accordingly, the conventional method needs a large number of masks for identification marks. As a result, manufacturing cost is significantly increased.
FIG. 1 is a top-view diagram showing a part of a chip having MASK ROM cells and an identification mark according to the prior art. In FIG. 1, number 10 represents a semiconductor substrate such as a silicon chip. Number 10a represents a device region on which MASK ROM cells is deposed. Number 12 represents an insulating region having an insulator such as field oxide (FOX) or shallow trench isolation (STI) oxide. Number 12a represents an identification mark such as xe2x80x9cV1xe2x80x9d. Numbers 14 and 16 represent a word line and a bit line of the MASK ROM respectively.
FIGS. 2a to 2e are section diagrams along the Axe2x80x94A line in FIG. 1 showing a conventional method of forming an identification mark for MASK ROM. In FIG. 2a, a semiconductor substrate 10 having an insulating region 12 and a device region 10a thereon is provided, and a word line 14 of the MASK ROM (not shown) is formed on the device region 10a. 
In FIG. 2b, after a code mask (not shown) is used for photolithography, a patterned resist layer 18 is formed on the insulating region 12 and the device region 10a to expose the word line 14 and a part of the device region 10a. Thereafter, ion implantation is performed to code the MASK ROM. The implanting depth of the device region 10a underlying the word line 14 is shallower than that of the exposed device region 10a near the word line 14. Mark xe2x80x9c+xe2x80x9d represents the doped element.
In FIG. 2c, after the patterned resist layer 10 is stripped, another patterned resist layer 20 is formed on the insulating region 12 and the device region 10a using a mark mask (not shown) to expose the insulating region 12. The top view pattern above the exposed insulating region 12 is xe2x80x9cV1xe2x80x9d.
In FIG. 2d, the exposed insulating region 12 is etched using the resist layer 20 as a mask to transfer the pattern xe2x80x9cV1xe2x80x9d to the insulating region 12 and an identification mark 12a is then formed. In FIG. 2e, the patterned resist layer 20 is stripped to complete the identification mark 12a fabrication.
As mentioned above, since mark masks are manufactured with different clients or different program codes, it is necessary to prepare a large number of mark masks for MASK ROM.
Seeking to solve this problem, a process for coding and code marking a read-only memory device is disclosed in U.S. Pat. No. 5,576,236. A mark is formed on a buffer layer using a code mask having identification mark pattern. This method only needs once photolithography and reduces the number of masks thereby reducing the cost. However, this method increases the number of process steps by forming an extra buffer layer. In addition, the mark is formed on the buffer layer, which is thin. Thus, the identification mark disappears or is difficult to recognize easily after the follow deposition steps.
An object of the present invention is to provide a method of forming an identification mark for read-only memory in which a common mask is used to reduce the number of masks used in the manufacturing process thereby reducing the manufacturing cost.
Therefore, the present invention provides a method of forming an identification mark for MASK ROM. The method uses code masks having identification mark patterns and a common mask to replace code masks and mark masks. Thus, the number of the masks for MASK ROM is reduced. Moreover, the identification mark is formed in the thicker field oxide or STI oxide to prevent disappearance of the mark. The mark can be recognized easily using an optical microscope (OM).
In accordance with the object of this invention, a method of forming an identification mark for read-only memory includes the steps of: providing a semiconductor substrate having an insulating region and a device region thereon; forming a first patterned resist layer on the insulating region and the device region by a first mask having an identification mark pattern over the insulating region; performing ion implantation to code in the device region; forming a second patterned resist layer on the first patterned resist layer by a second mask to expose the entire identification mark pattern of the first patterned resist layer only; etching the insulating region to transfer the identification mark pattern to the insulating region; and removing the first and the second patterned resist layers. Moreover, the insulating region is composed of field oxide or shallow trench isolation oxide and the device region is composed of at least one mask read-only memory. The first mask is a code mask further including a code pattern over the device region. The second mask is a common mask having an opening pattern to expose the identification mark pattern of the first resist layer only. The ion implantation can be performed using boron.
It is another object of the present invention is to provide a method of recognizing a chip having read-only memory cells in which a clear identification mark is formed on the insulating region, making recognition of the mark easy using optical equipment.
Accordingly, a method of recognizing a chip having read-only memory cells includes steps of: providing a semiconductor substrate having an insulating region and a device region thereon; forming a first patterned resist layer on the insulating region and the device region by a first mask having an identification mark pattern and a code pattern over the insulating region and the device region respectively; forming a second patterned resist layer on the first patterned resist layer by a second mask to expose the entire identification mark pattern of the first patterned resist layer only; etching the insulating region to transfer the identification mark pattern to the insulating region; removing the first and the second patterned resist layers to obtain an identification mark; and identifying the identification mark formed on the insulating region by optical equipment. Moreover, the method further includes the step of performing ion implantation to code in the device region after the first patterned resist layer is formed. The optical equipment can be an optical microscope.